Self-calibrating technique for x-ray imaging scanners

ABSTRACT

A mobile radiography apparatus has radio-opaque markers, each marker coupled to a portion of the mobile radiography apparatus, wherein each of the markers is in a radiation path that extends from an x-ray source or x-ray sources. A detector is mechanically uncoupled from the x-ray source or x-ray sources for positioning behind a patient. Processing logic is configured to calculate a detector position with relation to the x-ray source or x-ray sources according to identified marker positions in acquired projection images, and to reconstruct a volume image according to the acquired projection images.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation patent application of U.S. patentapplication Ser. No. 16/692,362, filed on Nov. 22, 2019, entitledSELF-CALIBRATING TECHNIQUE FOR X-RAY IMAGING SCANNERS, in the name ofLin, et al., which is a continuation patent application of U.S. patentapplication Ser. No. 15/971,213, filed on May 4, 2018, entitledSELF-CALIBRATING TECHNIQUE FOR X-RAY IMAGING SCANNERS, in the name ofLin, et al., which claims the benefit of U.S. Provisional ApplicationU.S. Ser. No. 62/507,288, provisionally filed on May 17, 2017, entitledSELF-CALIBRATING TECHNIQUE FOR X-RAY IMAGING SCANNERS, in the name ofLin et al., and U.S. Provisional Application U.S. Ser. No. 62/598,519,provisionally filed on Dec. 14, 2017, entitled SELF-CALIBRATINGTECHNIQUE FOR X-RAY IMAGING SCANNERS, in the names of Lin et al., whichare both hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure relates generally to the field of medical imaging, and inparticular to mobile radiographic imaging apparatus. More particularly,this disclosure relates to a simple and reliable self-calibratingtechnique for mobile X-ray imaging scanners.

BACKGROUND

Digital X-ray tomosynthesis is an imaging technique that enablesthree-dimensional imaging of a patient using a large-area digitaldetector typically used for conventional (single projection)radiography. A finite number of projection images over a limited angularrange, typically between 20° and 40°, are acquired by varying therelative orientations of the x-ray tube, patient and detector. This isusually accomplished by either moving both the detector and x-ray sourceor by fixing the position of the detector (source) and moving the x-raysource (detector). Three-dimensional data is reconstructed from thecaptured projections in the form of a number of slices through thepatient anatomy, each parallel to the detector plane. A consequence oflimited angular scanning is that the in-depth resolution is much lowerthan the in-plane resolution of the reconstructed object.

Reconstruction of volumetric data from a tomosynthesis system requiresknowledge of the underlying capture geometry, including the orientationand position of the detector, the movement and position of the sourceand potential patient motion. Precise geometric information of theimaging system (orientation of the X-ray detector, and locations of theX-ray tube and X-ray detector during 2D projection image acquisition)affect the image quality of the reconstructed images. Mismapping betweenobject space and the acquired 2D projection images can degrade spatialresolution and lead to image artifacts (e.g., ring artifacts).

In a conventional tomosynthesis system, many of the geometric variablesare known, as the detector position is precisely specified and therelationship between source and detector is also well established. Forstationary imaging scanners, acquisition geometry is fixed by themechanical coupling of source and detector, such as through a C-arm orother type of gantry arrangement. Calibration of this geometry isstraightforward, using a calibration phantom prior to image acquisition.

For a bed-side tomosynthesis system using mobile radiography apparatus,however, the detector is mechanically uncoupled from the source. Thus,the capture geometry is not fixed by system mechanics and can bedifficult to determine with the desired accuracy. Mobile x-ray imagingscanners used for tomosynthesis are designed for seriously ill patientswho cannot walk to, or stand in front of, stationary imaging scannershaving fixed geometry. Instead, in order to image these patients, thedetector is often manually positioned under the bed-ridden patient. Inthis acquisition environment, without the benefit of mechanically fixedsource-to-detector geometry, other approaches are needed in order toaccurately determine the geometry information in real time.

There is a need for a calibration utility that is suited to estimate thegeometry of a mobile radiography system used for tomosynthesis or other3D volume imaging and that overcomes the aforementioned limitations.

SUMMARY

An aspect of the present application is to advance the art ofradiography 3D volume imaging. Another aspect of this disclosure toaddress in whole or in part, at least the foregoing and otherdeficiencies in the related art. It is another aspect of thisapplication to provide in whole or in part, at least the advantagesdescribed herein.

Another aspect of the application is to provide methods and/or apparatusby which mobile radiography carts can additionally provide 3D imagingcapabilities with more accurate source/detector calibration.

Another aspect of the application is to provide methods and/or apparatusembodiments by which mobile radiography carts can acquire projectionimages and generate the reconstruction of three-dimensional tomographicand tomosynthesis images.

Another aspect of the application is to provide methods and/or apparatusembodiments by which mobile radiography carts can acquire x-ray 2Dprojection images and generate the reconstruction of two-dimensional orthree-dimensional volume images, where an imaging geometry of x-raysource positions to a radiographic detection array is not knownbeforehand.

These objects are given only by way of illustrative example, and suchobjects may be exemplary of one or more embodiments of the invention.Other desirable objectives and advantages inherently achieved by themethod and apparatus described herein may occur or become apparent tothose skilled in the art. The invention is defined by the appendedclaims.

According to one aspect of the disclosure, there is provided a mobileradiography apparatus comprising: a plurality of radio-opaque markers,each marker coupled to a portion of an x-ray head of the mobileradiography apparatus, wherein each of the markers is in a radiationpath that extends from at least one x-ray source; a detector that ismechanically uncoupled from the at least one x-ray source forpositioning behind a patient; and processing logic configured to: (i)calculate detector position with relation to the at least one x-raysource according to identified marker positions in acquired projectionimages; (ii) remove marker indications in the acquired projectionimages; (iii) reconstruct a volume image according to the acquiredprojection images; and (iv) display one or more portions of thereconstructed volume image.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the embodiments of the invention, as illustrated in theaccompanying drawings. The elements of the drawings are not necessarilyto scale relative to each other.

FIG. 1 is a diagram that shows a perspective view of a mobileradiography unit for use as a portable imaging system for tomosynthesisor other volume imaging.

FIG. 2 is a perspective view that shows a mobile radiography apparatusin position for volume imaging.

FIG. 3A is a perspective view that shows aspects of the image capturemetrics and behavior that relate to geometric calibration for mobileradiology apparatus when used for tomosynthesis and other volumeimaging.

FIG. 3B is a perspective view that shows additional aspects of the imagecapture metrics and x-ray head translation related to geometriccalibration for mobile radiology apparatus when used for tomosynthesisand other volume imaging.

FIG. 4 is a schematic cross-section of a conventional x-ray tube head.

FIG. 5 is a perspective view showing the function of a collimator light,which projects a pattern of light to assist registration of the patientto the source.

FIG. 6 is a schematic diagram showing components of a self-calibratingapparatus for a mobile radiography apparatus.

FIG. 7 is a logic flow diagram that shows a sequence for calibration ofa mobile radiography apparatus having a detector that is notmechanically coupled to the x-ray source.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following is a description of exemplary embodiments of theinvention, reference being made to the drawings in which the samereference numerals identify the same elements of structure in each ofthe several figures. Where they are used, the terms “first”, “second”,and so on, do not necessarily denote any ordinal or priority relation,but may be used for more clearly distinguishing one element or timeinterval from another.

Portable radiographic systems are routinely used in hospitals. Comparedto standard projection radiography, volume imaging apparatus such astomosynthesis apparatus provide improved depiction of fine details notvisible in normal radiographs due to overlying structures. Thesebenefits provide an impetus to develop portable volume imaging systemsthat can be utilized in the intensive care unit, emergency department,and operating rooms, where moving the patient is either impracticable orill-advised due to the risk of harm to the patient.

The image quality of the reconstruction depends upon accurate knowledgeof the acquisition scan geometry, including spatial and angularpositions of the x-ray source and detector for each projection.Uncertainties in the scan geometry can lead to artifacts and/or blurringin the reconstructed object. The development of portable volume imagingsystems has been hampered by difficulties in accurately determining theacquisition scan geometry. There remains a need for improved X-rayvolume imaging systems that can be made portable and still providereliable clinical images and data.

Reference is hereby made to an article by S. Quadah, J. W. Stayman, G.J. Gang, A. Uneri. T. Ehtiati, and J. H. Siewerdsen entitled“Self-calibration of cone-beam CT geometry using 3D-2D imageregistration” in Phys. Med. Biol. 2016, Apr. 7; pp. 2613-2632.

Reference is made to commonly assigned U.S. Pat. No. 8,821,017 (Lalena)entitled PROJECTOR AS COLLIMATOR LIGHT, incorporated herein in itsentirety by reference.

FIG. 1 is a diagram that shows a perspective view of a mobileradiography unit that can use portable radiographic detectors or flatpanel detectors that are mechanically uncoupled from the radiationsource according to embodiments of the application. The exemplary mobilex-ray or radiographic apparatus of FIG. 1 can be employed for digitalradiography (DR) and/or tomosynthesis or tomographic imaging. As shownin FIG. 1, a mobile radiography apparatus 100 can include a moveabletransport frame 120 that includes a first display 110 and an optionalsecond display 110′ to display relevant information such as obtainedimages and related data. As shown in FIG. 1, the second display 110′ canbe pivotably mounted at the x-ray source, x-ray tube, or x-ray head 140to be viewable/touchable from a 360 degree area.

The displays 110, 110′ can implement or control (e.g., touch screens)functions such as generating, storing, transmitting, modifying, andprinting of the obtained image(s) and can include an integral orseparate control panel (not shown) to assist in implementing functionssuch as generating, storing, transmitting, modifying, and printing ofobtained image(s). An optional touchpad 150 allows support functionssuch as operator identification.

For mobility, the mobile radiographic apparatus 100 can have one or morewheels 115 and one or more handle grips 125, typically provided atwaist-level, arm-level, or hand-level, that help to guide the mobileradiographic apparatus 100 to its intended location. A self-containedbattery pack (e.g., rechargeable) can provide source power, which canreduce or eliminate the need for operation near a power outlet. Further,the self-contained battery pack can provide for motorized transport.

For storage, the mobile radiographic apparatus 100 can include anarea/holder for holding/storing one or more digital radiographic (DR)detectors or computed radiography cassettes. The area/holder can bestorage area (e.g., disposed on the frame 120) configured to removablyretain at least one digital radiography (DR) detector. The storage areacan be configured to hold a plurality of detectors and can also beconfigured to hold one size or multiple sizes of DR detectors.

Still referring to FIG. 1, a control logic processor 130 provides thecontrol logic for image processing and identification of the position ofthe detector relative to the x-ray source. Image processing can beprovided by a processor that is part of mobile radiography apparatus 100itself, or can be provided by one or more external computers and otherprocessors networked in signal communication with mobile radiographyapparatus 100.

Mounted to frame 120 is a support column 135 that supports x-ray head140, also called an x-ray tube, tube head, or generator, that includesthe x-ray source and that can be mounted to the support member 135. Inthe embodiment shown in FIG. 1, the support member (e.g., column 135)can include a second section that extends outward a fixed/variabledistance from a first section where the second section is configured toride vertically up and down the first section to the desired height forobtaining the image. In addition, the support column is rotatablyattached to the moveable frame 120. In another embodiment, the tube head140 can be rotatably coupled to the support column 135. In anotherexemplary embodiment, an articulated member of the support column thatbends at a joint mechanism can allow movement of the x-ray tube head 140over a range of vertical and horizontal positions. Height settings forthe x-ray tube head 140 can range from low height for imaging feet andlower extremities to shoulder height and above for imaging the upperbody portions of patients in various positions.

The perspective view of FIG. 2 shows mobile radiography apparatus 100 inposition for tomosynthesis or tomographic imaging. A patient 14 is lyingflat on a bed or stretcher with a detector 20 fitted behind the patient14 with respect to the x-ray source. There is no inherent mechanicallinkage or alignment between x-ray head 140 and detector 20 andconventional calibration of source/detector geometry prior to exposureis not feasible.

The perspective views of FIGS. 3A and 3B show aspects of the imagecapture metrics and behavior that relate to geometric calibration formobile radiology apparatus 100 when used for tomosynthesis or othervolume imaging. A source-to-image distance SID can be approximated basedon known factors, such as bed height and column height for radiographyapparatus 100. Even where the SID can be accurately identified, however,there are additional calibration metrics that must be known. Theseinclude factors such as skew of the detector 20 relative to the x-raysource as shown in FIG. 3A and the relative travel path of the sourceand detector as successive images are acquired, as shown for lineartravel by a transport apparatus 30 in FIG. 3B.

The cutaway cross-sectional schematic view of FIG. 4 shows conventionalx-ray tube head 140 having an x-ray radiation source 50 with acollimator 22 and a collimator light 26. Collimator 22 typically has twocollimator sections 76 a and 76 b, each with blades positioned forshaping the output radiation beam. A collimator light 26, typically alight bulb or light emitting diode (LED) or other solid-state lightsource, mounts inside collimator 22 and serves as a guide for aimingorientation of the x-ray head 140. A mirror 24, essentially transparentto x-rays but reflective to visible light, combines the light path ofcollimator light 26 with the radiation path R of x-ray beam 32 thatextends from the x-ray source, so that the cross-sectional area of thelight beam from collimator light 26 matches the cross-sectional area ofthe collimated radiation beam that is emitted from x-ray head 140.

The perspective view of FIG. 5 shows the function of collimator light26, which typically projects a pattern of light to assist registrationof the patient to the source so that the collimated beam is directed tothe subject region that is properly within the perimeter of the detector20. As has been emphasized, however, it is generally not feasible toidentify the exact position and skew orientation of detector 20 relativeto the beam from x-ray head 140 for mobile radiography applications, forreasons such as detector placement and condition of the patient.

According to embodiments of the present disclosure, geometriccalibration of the source and detector is performed using the acquiredradiography image content. Radio-opaque markers are disposed withinx-ray head 140, at fixed positions along radiation path R. The imagecontent that is acquired at the x-ray detector 20 includes the markers.The position of the markers in the acquired x-ray image relates directlyto the relative geometry of the x-ray source 50 and detector 20 and canbe used to calculate this geometry with sufficient accuracy to provide afaithful reconstruction of depth information for the imaged anatomy.

Referring to the simplified schematic diagram of FIG. 6, mirror 24 ofx-ray head 140 is shown in relation to the position of collimator light26 and x-ray source 50. Mirror 24 has been modified to include a set ofradio-opaque markers 40, each marker 40 disposed in path R of the x-rayradiation directed toward detector 20. Because each marker isradio-opaque, the markers 40 are detected in the x-ray image contentfrom detector 20. The pattern of markers 40 that are imaged by detector20 and the overall geometry of the detected pattern can providesufficient information for an accurate calibration of detector 20 tox-ray source 50.

It should be noted that, while mirror 24 can be a convenient vehicle formounting of the radio-opaque markers within x-ray head 140, othercomponents that lie in the radiation path R can alternately be used,such as filters, for example. The markers can be coupled to a suitableradio transparent support surface or to some other feature that is inthe path of the radiation beam and collimator light.

There are a number of considerations that relate to marker use forgeometric calibration of a mobile radiography apparatus, including thefollowing:

(i) Marker positioning along the periphery of the radiation field.Peripheral positions of markers 40 with respect to the detector imagingarea are generally advantageous. Anatomical information of interest isgenerally centered in the image area; markers 40 along the edges oroutside of the imaged area are less likely to interfere with 3Dreconstruction.

(ii) Fixed positioning. Marker positions are fixed in the housing of thex-ray head 140 and do not change with adjustment of head 140 position.If markers 40 are provided on the mirror, the mirror should be in afixed position within the head.

(iii) Calibration of markers 40 to head 140. An initial geometriccalibration of markers 40 position to the head 140 and x-ray source 50is performed as a setup procedure for the mobile radiography apparatus,prior to use of markers 40 for source/detector calibration for a patientimage.

(iv) Shadows. Markers 40 generate shadows in the acquired projectionimage. Additional image processing steps are needed in order to removethe marker shadows following geometric calibration. Well-known imageprocessing procedures such as segmentation, interpolation, andin-painting can be employed to compensate for shadow effects.

(v) High magnification factor. The geometry magnification factor formarkers is significant. Thus, the markers 40 themselves should be madeas small and distinct as possible.

(vi) Marker shape. Specially shaped markers 40 facilitate markerdetection and removal from the projection images. For example, circular,triangular, or cross-shaped markers 40 may be advantaged. Although themarker shape may increase marker size, distinctive shapes can help tosimplify detection of the center of the marker, allowing readyidentification and removal.

(vii) Adjustable collimators. Depending on the design of x-ray head 140,collimators 22 may be adjustable. This factor must be taken into accountfor marker design and positioning. A specific collimator position maywork best for subsequent detection and removal of marker effects.

Calibration of marker position (item (iii) above) can be performed byestablishing precise positional coordinates for detector 20 relative tox-ray source 50 and acquiring image content from two or more exposuresalong the travel path of x-ray head 140. Calibration of marker positioncan then be calculated from positional and movement information from theresulting sequence of projection images, using well known methods oftriangulation and projective geometry.

Using the initially calibrated marker 40 position with respect to x-raysource 50, metrics such as SID, skew, and planar orientation of thedetector 20 can be readily computed using well-known projective geometrycalculations.

The logic flow diagram of FIG. 7 shows a sequence for calibration of amobile radiography apparatus having a detector that is not mechanicallycoupled to the x-ray source and wherein (i) the apparatus is configuredwith markers and (ii) marker calibration to the x-ray source has beenperformed. A set of 2D projection images of the subject is acquired forprocessing in an image acquisition step S710. Marker positions in theprojection images are then identified in an identification step S720. Acalculation step S730 then calculates detector/source geometry based onidentified marker position, using well known projective geometryprocessing. A cleanup step S740 is then executed, in which markercontent is removed from the projection images. Cleanup step S740 can useinterpolation, in-painting, and other well known processes in order torestore image content and remove marker shadows, as noted previously.This can be readily accomplished, since the markers have the sameposition in each acquired projection image. A reconstruction step S750can then be executed in order to generate the 3D volume image fromcorrected 2D projection images. Reconstruction methods for tomographyand tomosynthesis imaging are well known and include filtered backprojection (FBP) and iterative reconstruction methods, allowing a numberof processing sequences to be used for volume image reconstruction. 2Dslices or other portions or projections of the generated 3D volume imagecan then be extracted and displayed in a display step S760.

According to an embodiment of the present disclosure, only a portion ofthe acquired projection images in a tomosynthesis series or tomographyseries are analyzed for marker position detection and calculation ofsource/detector geometry in step S730.

Using radio-opaque markers embedded along the radiation path andexecuting the process outlined with respect to FIG. 7, an embodiment ofthe present disclosure enables source/detector geometry to be calculatedfrom the set of projection images that is acquired. Thus, embodiments ofthe present disclosure are advantageous for calibration use with mobileradiography apparatus and other radiography systems in which the imagingdetector is mechanically uncoupled from the radiation source.

Markers 40 can be formed of lead or other radio-opaque material,including metals such as tungsten. Markers can be formed into beads orinto some other suitable shape, such as a cross or circle, a square, atriangle, or some other shape and can be adhered, imprinted bydeposition onto a radio-transparent surface, or otherwise coupled to amirror, filter, or other permanent feature of the x-ray head 140.Markers 40 can be coupled to a radio-transparent support feature, suchas a glass or plastic surface or feature, that is in the path ofcollimated energy from the x-ray source.

The following references are cited:

-   Yuan Lin and Ehsan Samei. “A FAST POLY-ENERGETIC FBP ALGORITHM”,    Physics in Medicine and Biology 59 (2014) pp. 1655-1678;-   Yuan Lin and Ehsan Samei, “AN EFFICIENT POLYENERGETIC SART (pSART)    RECONSTRUCTION ALGORITHM FOR QUANTITIVE MYOCARDIAL CT PERFUSION”,    Medical Physics, 41 (2) February 2014. pp. 021911-1 to 021911-14;-   F. Edward Boas and Dominik Fleischmann, “CT ARTIFACTS: CAUSES AND    REDUCTION TECHNIQUES”, Imaging Med. (2012) 4 (2), 229-240, pp. 1-19;-   US 2008/0095302 (Ruhrnschopf) titled “METHOD FOR HARDENING    CORRECTION IN MEDICAL IMAGING”;-   WO 2016/003957 (Lin) titled “SPECTRAL ESTIMATION AND POLY-ENERGETIC    RECONSTRUCTION METHODS AND X-RAY SYSTEMS” published on Jan. 7, 2016.

A computer program product may include one or more storage medium, forexample; magnetic storage media such as magnetic disk (such as a floppydisk) or magnetic tape; optical storage media such as optical disk,optical tape, or machine readable bar code; solid-state electronicstorage devices such as random access memory (RAM), or read-only memory(ROM); or any other physical device or media employed to store acomputer program having instructions for controlling one or morecomputers to practice the method according to the present invention.

The invention has been described in detail, and may have been describedwith particular reference to a suitable or presently preferredembodiment, but it will be understood that variations and modificationscan be effected within the spirit and scope of the invention. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative and not restrictive. The scope of the invention isindicated by the appended claims, and all changes that come within themeaning and range of equivalents thereof are intended to be embracedtherein.

What is claimed is:
 1. A mobile radiography apparatus comprising: anx-ray source; a plurality of radio-opaque markers disposed in aradiation path extending from the x-ray source to a radiographicdetector mechanically uncoupled from the x-ray source; and a processorprogrammed to calculate a position of the detector relative to the x-raysource according to positions of the radio-opaque markers in acquiredprojection images, and to reconstruct a volume image according to theacquired projection images.
 2. The apparatus of claim 1, wherein one ormore of the plurality of radio-opaque markers have a spherical shape. 3.The apparatus of claim 1, wherein one or more of the plurality ofradio-opaque markers is formed by depositing radio-opaque material ontoa surface.
 4. The apparatus of claim 1, further comprising a collimatorlight and a mirror, wherein one or more of the plurality of radio-opaquemarkers is coupled to the mirror, and wherein the mirror is disposed inan illumination path of the collimator light.
 5. The apparatus of claim1, further comprising a radiolucent support disposed in the radiationpath, wherein one or more of the plurality of radio-opaque markers iscoupled to the radiolucent support.
 6. The apparatus of claim 1, furthercomprising a transport apparatus for translating the x-ray source alonga path to acquire a sequence of images of the patient using the x-raysource at different positions along the path.
 7. The apparatus of claim1, wherein the control logic processor is further programmed to removean image of the radio-opaque markers from the acquired projectionimages.
 8. The apparatus of claim 1, further comprising an x-ray head,wherein the plurality of radio-opaque markers are rigidly attached tothe x-ray head.
 9. A mobile radiography apparatus comprising: an x-raysource; a plurality of radio-opaque markers disposed in an x-ray beamextending from the x-ray source to a radiographic detector mechanicallyuncoupled from the x-ray source; and a processor programmed to calculatea position of the detector relative to the x-ray source according topositions of the radio-opaque markers in projection images captured bythe detector, and to reconstruct a volume image according to thecaptured projection images.
 10. The apparatus of claim 9, wherein one ormore of the plurality of radio-opaque markers have a spherical shape.11. The apparatus of claim 9, wherein one or more of the plurality ofradio-opaque markers is formed by depositing radio-opaque material ontoa surface.
 12. The apparatus of claim 9, further comprising a collimatorlight and a mirror, wherein one or more of the plurality of radio-opaquemarkers is coupled to the mirror, and wherein the mirror is disposed inan illumination path of the collimator light.
 13. The apparatus of claim9, further comprising a radiolucent support disposed in the x-ray beam,wherein one or more of the plurality of radio-opaque markers is coupledto the radiolucent support.
 14. The apparatus of claim 9, furthercomprising a transport apparatus for translating the x-ray source alonga path to capture a sequence of images of a patient using the x-raysource at different positions along the path.
 15. A method of operatinga mobile radiography apparatus having an x-ray source, the methodcomprising: positioning a plurality of radio-opaque markers in aradiation beam extending from the x-ray source to a subject; using thex-ray source and a detector, acquiring a sequence of projection imagesof the subject; processing each of the acquired projection images of thesubject to identify a position of one or more of the radio-opaquemarkers within the acquired projection images; using the identifiedposition of the one or more radio-opaque markers, calculating a positionof the detector with respect to the x-ray source; and reconstructing avolume image of the subject using the acquired projection imagesaccording to the calculated position of the detector.
 16. The method ofclaim 15, further comprising processing the acquired projection imagesto remove the one or more radio-opaque markers within the acquiredprojection images.
 17. The method of claim 15, wherein the step ofprocessing the acquired projection images to remove the one or moreradio-opaque markers comprises using in-painting or interpolation. 18.The method of claim 15, further comprising rigidly attaching theplurality of radio-opaque markers to a portion of the mobile radiographyapparatus.
 19. The method of claim 18, wherein the step of rigidlyattaching the plurality of radio-opaque markers comprises using a rigidradiolucent support arm.
 20. The method of claim 15, further comprisingforming one or more of the plurality of radio-opaque markers using leador tungsten in a shape of a sphere, a cross, a circle, a square, or atriangle.